Exposure of Arabidopsis thaliana to excess Zn reveals a Zn-specific oxidative stress signature

Abstract

Zinc (Zn) is an essential micronutrient for plants, but accumulation of excess Zn causes oxidative stress, even though the element is not redox-active. An oxidative stress signature, consisting of multiple oxidative stress related parameters, is indicative of disturbance of redox homeostasis and signaling, but has not been determined after exposure to excess Zn. To reveal general and Zn-specific effects, changes in oxidative stress related gene expression, enzyme activities and metabolites were determined after 24 h exposure of Arabidopsis thaliana plants to a concentration range of 0, 100,250 or 500 mu M excess ZnSO4, and the oxidative stress signature compared to the ones reported previously for cadmium or copper exposure in the same experimental system. Zn accumulated in both roots and leaves, which led to a disturbed redox homeostasis as evident from increased H2O2 levels and altered glutathione redox state. In roots, magnesium uptake was affected and glutathione levels decreased due to reduced glutathione reductase activity and increased phytochelatin production. In leaves, mRNA levels of all chloroplast-localized superoxide dismutases (SOD) were downregulated, indicating that the converse regulation by Cu availability of FeSOD (FSD) and Cu/ZnSOD genes (CSDs) was disturbed. Furthermore, an opposite stress-dependent transcriptional regulation of the different loci of a miRNA species was observed: excess Zn repressed primary transcripts of the MIR398a gene, but increased MIR398b and MIR398c transcription. Also, accumulation of mRNA was inhibited for CSD2, but stimulated for CSD1. MiR398 has been reported to downregulate CSD1 and CSD2 mRNA. Here, the opposite effect on CSD1 and CSD2 mRNA accumulation under Zn stress in the leaves could be explained if CSD1 in the leaves would be regulated exclusively by miR398a, and not by miR398b/c under these conditions. The results support the existence of Zn-specific signal transduction pathways influencing anti-oxidative responses and are useful as a starting point for genetic screens identifying upstream metal-specific sensing and signaling mechanisms. (C) 2012 Elsevier B.V. All rights reserved.